1. Field of the Invention
The present invention relates to a surface acoustic wave-semiconductor composite device using an interaction between surface acoustic waves and surface charges in a semiconductor, in particular, to a surface acoustic wave convolver.
2. Description of the Prior Art
Recently, electric waves for communication are used widely according to the development of mobile communication technology. Then, shortage of channels, cross talk, eavesdropping and the like become large problems. In order to solve these problems, various new communication methods are proposed. Code division multiple access (CDMA) method is one of potential communication methods which have been studied and developed because it can increase a channel number by a factor of one digit or more in a same frequency band. The CDMA method uses a spread-spectrum communication technology, wherein a receive terminal usually includes a convolver for a convolution operation of input signals with standard signals in order to demodulate signals.
Surface acoustic wave convolvers are known which combines a surface acoustic wave device and a semiconductor element, and they are classified into a monolithic type, wherein a semiconductor substrate and a piezoelectric plate are layered integrally as a one body, and a separation medium structure type, wherein a gap is provided between a piezoelectric plate and a semiconductor plate opposing each other. A surface acoustic wave convolver of monolithic type is described, for example, in a paper by B. T. Khuri-Yakub et al. (Applied Physics Letters, Vol. 25, 188-190 (1974)), while a surface acoustic wave convolver of separation medium type is described in a paper by G. S. Kino et al. (Journal of Applied Physics, Vol. 44, 5219-5221 (1973)).
FIG. 1 shows a representative metal-insulator-semiconductor (MIS) structure of a surface acoustic wave convolver of monolithic type. A semiconductor plate 21 made of silicon or the like and a piezoelectric plate 22 made of ZnO, AlN or the like are layered with an insulator layer 26 provided at an interface between them. The insulator layer 26 is made of SiO.sub.2 prepared for example with sputtering, and the piezoelectric plate 22 is also prepared with sputtering. Signal input electrodes 23, 23' of interdigital transducer (IDT) and an output electrode 24 are provided on the piezoelectric plate 22, while a ground electrode 25 is provided on the semiconductor plate 21.
Next, an operation of the surface acoustic wave convolver is explained. Signals are supplied to the two signal input electrodes 23, 23' of interdigital transducer and surface acoustic waves propagates along the interface and interact with each other below the output electrode 24. The surface acoustic waves cause displacement of charges of the semiconductor plate 21 along the interface, so that they change a capacitance of a depletion region in the semiconductor plate 21. Convolution signals are taken out from the output electrode 24 by using nonlinearity of voltage dependence of the capacitance of the depletion layer.
The surface acoustic wave convolver of monolithic type has a high sensitivity and good characteristics. It is also known that the characteristics can be improved by forming a semiconductor layer of low concentration or a semiconductor layer of various kinds of structures with epitaxial crystal growth (for example, Japanese Patent laid open Publication 63-197111/1988).
In the prior art surface acoustic wave convolver, materials which can be used as a piezoelectric thin film is limited. It is required that a material of good piezoelectric properties is formed on a semiconductor substrate of silicon or the like or on an insulator layer of SiO.sub.2 or the like formed on the semiconductor substrate with a thin film technique such as sputtering, chemical vapor deposition, vacuum deposition, molecular beam epitaxial growth or the like. Such a material is substantially limited to ZnO and AlN. Lithium niobate and lithium tantalate having a large electromechanical coupling factor and large nonlinearity, and quartz and lithium borate having very small temperature dependence cannot be used for the piezoelectric plate 22. Though a thin film of lithium niobate is tried to grow with sputtering, a resultant thin film has characteristics much worse than a single crystal of lithium niobate. Generally, properties such as piezoelectric property which are closely related to the crystallinity of a material are better for a single crystal than for a layer prepared with a thin film technique. This situation is similar to other materials such as lithium tantalate, lithium borate and quartz.
Properties propagated by surface acoustic waves are very sensitive to the density, crystallinity and the like of the piezoelectric material. Therefore, a productivity is also a problem because it is difficult to reproduce the density, crystallinity and the like with a good yield.
A reverse structure is also known where a semiconductor thin film of InSb or the like is formed on a single crystal plate of piezoelectric material with vacuum deposition or the like. However, this structure deteriorates semiconductor characteristics largely. Because the monolithic convolver uses a change in a capacitance of the depletion region at the interface of the semiconductor substrate, it is necessary to keep the electronic states at the interface good. However, a semiconductor thin film formed on a substrate of a different material with a thin film technique such as vacuum deposition, sputtering, chemical vapor deposition or the like becomes a polycrystal or an amorphous crystal. Therefore, an obtained film has very many surface levels, or the electronic states at the interface cannot be controlled well sufficiently. Therefore, this structure also cannot produce a convolver of good characteristics. On the other hand, it is not desirable practically to prepare a thick piezoelectric plate as an insulator because an operation voltage becomes higher if compared with a convolver with a thick semiconductor having a low resistance.
On the other hand, if an adhesive such as an organic material or glass is used to combine a semiconductor plate and a piezoelectric plate to form a convolver, it has several disadvantages. First, because the thickness of the adhesive layer is usually of an order of micrometers or 1-5 micrometers, a voltage change due to surface acoustic waves cannot be transmitted to the semiconductor plate efficiently. Further, because the thickness cannot be controlled precisely, convolution signals are deteriorated due to the nonuniform thickness. Further, because the thickness of the adhesive layer varies in each production process, the reproducibility of the convolver cannot become good. Second, the adhesive agent itself is not stable thermally and chemically. Therefore, the temperature dependence of the characteristics is a problem. Thermal stability for a long time and reliableness on mechanical stability are not sufficient. Further, an application of an adhesive agent on the semiconductor surface generates unnecessary surface levels, and this deteriorates properties of a convolver.
FIG. 2 shows a representative structure of a separation medium structure surface acoustic wave convolver. A piezoelectric plate 22' having large nonlinearity, made of for example lithium niobate single crystal, and a semiconductor plate 21 made of silicon are separated by a narrow gap 27 with spacers 28, made of silicon oxide films, placed at positions which do not affect the propagation of surface acoustic waves as much as possible. Adhesive agents 28 are also provided to adhere the semiconductor plate 21' and the piezoelectric plate 22' at positions which do not affect the propagation of surface acoustic waves as much as possible. A ground electrode 25 is provided on a surface of the piezoelectric plate 22', while signal input electrodes 23, 23' or interdigital transducer (IDT) are provided on the other opposite surface of the piezoelectric plate 22'. An output electrode 24 is provided on the semiconductor plate 21. Posts of about 5 .mu.m of diameter and height of 100 nm prepared with etching may also be used instead of the spacer 28 in order to keep the gap 27 to have a constant distance between the piezoelectric plate 22' and the semiconductor plate 21.
Next, an operation of the convolver is explained. Signals are supplied to the two signal input electrodes 23, 23' or interdigital transducers and surface acoustic waves propagate along the surface of the piezoelectric plate 22' and interact with each other below the output electrode 24. The surface acoustic waves cause displacement of charges of the semiconductor plate 21 along the surface. This displacement generates a change in potential at the surface of the semiconductor plate 21. Convolution signals are taken out from the output electrode 24 by using nonlinearity of the interaction.
A separation medium structure surface acoustic wave convolver has a high sensitivity and good characteristics because nonlinearity of a lithium niobate single crystal having large nonlinearity and a semiconductor surface is used.
In a separation medium structure surface acoustic wave convolver described above, the sensitivity depends on the distance of the gap 27 very much. Therefore, fixing of the spacers 28 or the like is an important problem to keep the gap to have a constant distance. That is, in the structures using the spacers, the etching posts or the like, an adhesive agent is used for binding, or screws or threads are used for mechanical fixing. However, characteristics change easily due to thermal or mechanical stress. If an adhesive made of an organic material is used, the distance of the gap cannot be controlled precisely, and if gas is generated on heating, characteristics are effected by the adsorption of gas. If an adhesive made of an inorganic material such as solder or glass is used, the distance of the gap cannot be controlled precisely. Then, though a separation medium structure surface acoustic wave convolver has a high sensitivity, it is not used substantially practically.
Beside the above-mentioned convolvers, there are known, there are known similar devices. In such devices, a piezoelectric plate and a semiconductor plate are layered or opposed with each other with a very small gap between them, and surface acoustic waves are propagated on a surface of the piezoelectric plate opposing the semiconductor plate to interact with charges in the semiconductor plate to generate various effects. Such devices are called as surface acoustic wave-semiconductor composite device in the description. For example, a surface state memory in surface acoustoelectric correlator is described in a paper by A. Bers and J. H. Cafarella (Applied Physics Letters, Vol. 25, 133-135 (1974)), and a surface acoustic wave amplifier is described in a paper by K. Yamanouchi and K. Shibayama (J. Applied Physics, Vol. 43, 856-862 (1972)).